Geochronology and Geochemistry of Paleoproterozoic Mafic Rocks in Northern Liaoning and Their Geological Significance

: Petrological, geochronological, and geochemical analyses of mafic rocks in northern Liaoning were conducted to constrain the formation age of the Proterozoic strata, and to further study the source characteristics, genesis, and tectonic setting. The mafic rocks in northern Liaoning primarily consist of basalt, diabase, gabbro, and amphibolite. Results of zircon U-Pb chronology reveal four stages of mafic magma activities in northern Liaoning: the first stage of basalt (2209 ± 12 Ma), the second stage of diabase (2154 ± 15 Ma), the third stage of gabbro (2063 ± 7 Ma), and the fourth stage of magmatic protolith of amphibolite (2018 ± 13 Ma). Combined with the unconformity overlying Neoproterozoic granite, the formation age of the Proterozoic strata in northern Liaoning was found to be Paleoproterozoic rather than Middle Neoproterozoic by the geochronology of these mafic rocks. A chronological framework of mafic magmatic activities in the eastern segment of the North China Craton (NCC) is proposed. The mafic rocks in northern Liaoning exhibit compositional ranges of 46.39–50.33 wt% for SiO 2 , 2.95–5.08 wt% for total alkalis (K 2 O + Na 2 O), 6.17–7.50 wt% for MgO, and 43.32–52.02 for the Mg number. TiO 2 contents lie between 1.61 and 2.39 wt%, and those of MnO between 0.17 and 0.21 wt%. The first basalt and the fourth amphibolite show low total rare earth element contents. Normalized against primitive mantle, they are enriched in large ion lithophile elements (Rb, Ba, K), depleted in high field strength elements (Th, U, Nb, Ta, Zr, Ti), and exhibit negative anomalies in Sr and P, as well as slight positive anomalies in Zr and Hf. The second diabase and the third gabbro have similar average total rare earth element contents. The diabase shows


Introduction
As one of the oldest cratons in the world, the North China Craton (NCC) is a product of the amalgamation of multiple micro landmasses during the Late Neoarchean to Paleoproterozoic [1][2][3][4][5][6][7].The researchers have different explanations on the tectonic evolution of the NCC.Four Archean landmasses and three Paleoproterozoic orogenic belts have been identified by some researchers (Figure 1c; [1,2,7]), and the aggregation process is summarized as follows: (1) In the western part of the NCC, the Yinshan Block and the Ordos Block converged along the Khondalite Belt at 1.95 Ga, forming a unified land block known as the Western Block.(2) In the eastern part, the Longgang Block and the Liaonan-Rangnim Block converged along the Jiao-Liao-Ji Belt (JLJB) at 1.92 Ga resulting in the formation of the Eastern Block.(3) The final collision occurred at 1.85 Ga between the Eastern and Western Blocks along the Trans-Central Orogenic Belt, leading to the final amalgamation of the NCC into a unified Precambrian continent [1,2,7].However, other scholars have a different understanding of the convergence process of the NCC, with regard to the amalgamation of smaller tectonic units into larger continental landmasses at the end of the Archean and into the Paleoproterozoic with the formation of the Columbia Continent [3,4].The Western Block collided with the arc-modified margin of the composite Eastern Block at 2.43 Ga leading to the formation of the Central Orogenic Belt with the imbricated arc and fore-arc ophiolitic mélanges.The northern margin of the craton was modified to become an Andean-style arc from 2.3 Ga to 1.9 Ga soon after this collision, and numerous magmatic rocks, volcanic and volcaniclastic rocks, and thick clastic sediments occurred in the continental-margin arc and retro-arc foreland basins.From about 1.88 to 1.79 Ga, the Columbia/Nuna Continent collided with the NCC along the northern margin of the craton resulting in the formation of the Inner Mongolia-Northern Hebei Orogen (IMNHO) [3,4,8,9].Thus, the Paleoproterozoic geological bodies exposed within the orogenic belt are the key to reconstructing the tectonic evolution of the NCC.
In addition to the JLJB, many Paleoproterozoic magmatic and metamorphic events have been identified in the Qingyuan terrane of the northern margin of the NCC [30][31][32].The Paleoproterozoic Inner Mongolia-Northern Hebei Orogen, as defined by Kusky et al. (2007), has been extended to the northern Liaoning but lacks direct geochronological evidence [4,5,9].A suite of low-grade metamorphic rocks consisting of dolomite, sandstone, and siltstone has been exposed in northern Liaoning.Due to a lack of direct evidence of geochronology, its deposition age has been controversial, and it is temporarily classified as Mesoproterozoic based solely on lithological comparisons [33].Large-scale mafic rocks intrude into this stratigraphic sequence.According to whole-rock K-Ar dating results, these mafic rocks are considered to have formed in the Early Triassic [33].However, Minerals 2024, 14, 717 3 of 30 during the field investigation, the authors found that these mafic rocks intrude into the dolomite, or are enveloped by marble, and have undergone high erosion, alteration, and metamorphism, similar to the large-scale Paleoproterozoic mafic dykes developed in the Liaoyang-Haicheng area in the south of the Longgang Block, rather than the Mesozoic mafic dykes [13,14].Meanwhile, a 2.12 Ga metabasic dyke has recently been reported in Qingyuan [30].Therefore, are the mafic rocks in northern Liaoning also products of the Paleoproterozoic magmatic events?What is the relationship with the mafic rocks in the JLJB and the tectonic setting in which they were formed?The composition of maficultramafic rocks can directly reflect their sources and tectonic settings.Thus, the mafic rocks in northern Liaoning were selected for petrographic, geochronological, and geochemical studies to constrain the deposition age of the Proterozoic sediments and reconstruct the Paleoproterozoic tectonic evolution in this area.In addition to the JLJB, many Paleoproterozoic magmatic and metamorphic events have been identified in the Qingyuan terrane of the northern margin of the NCC [30][31][32].The Paleoproterozoic Inner Mongolia-Northern Hebei Orogen, as defined by Kusky et al. (2007), has been extended to the northern Liaoning but lacks direct geochronological evidence [4,5,9].A suite of low-grade metamorphic rocks consisting of dolomite, sandstone,

Geological Setting and Sample Descriptions 2.1. Geological Setting
The Precambrian basement of Liaodong Peninsula, located in the northeastern segment of the NCC, is composed of the Archean crystalline basement including the Liaonan-Nangrim Block in the southeast and the Longgang Block in the northwest, and the Paleoproterozoic JLJB in the center (Figure 1a; [1]).Archean to Paleoproterozoic supracrustal and granitoid rocks are exposed in the Liaonan-Nangrim Block in the southern Liaoning [34].The Paleoproterozoic JLJB between these two blocks mainly contains metavolcano-sedimentary successions and granitic to mafic intrusions that were metamorphosed to greenschistamphibolite facies [13][14][15][16][17][18][19][20].The Longgang Block in the northern Liaoning exhibits extensive occurrences of tonalite-trondhjemite-granodiorite rocks (TTGs) [35][36][37] and basic volcanic rock [38].These rocks, which date back approximately 3.8 Ga, have been discovered in the Anshan area.Furthermore, various zircons of magmatic, xenocrystal, and detrital origins, with ages ≥ 3.0 Ga, have been identified in this region [39].
The Proterozoic geological units are mainly exposed in the northern Liaoning, located from north of Shenyang-Fushun to Kaiyuan, and from Tieling to Wangxiaopu in the east.Based on rock composition, the stratigraphy is compared with the Changcheng and Jixian Systems' strata in western Liaoning, and is placed in the Mesoproterozoic.According to the current classification scheme, it is divided from bottom to top into the Daposhan Formation, Kangzhuangzi Formation, and Guanmenshan Formation of the Changcheng System, the Tongjiajie Formation, Hutouling Formation, Erdaogou Formation, Shimen Formation, and Yangshitun Formation of the Jixian System, the Yubeigou Formation and Yaomotaichong Formation of Neoproterozoic, and the Yintun Formation of the Nanhua System.These strata have undergone low-grade metamorphism and overlaid Archean gneiss with angular unconformity [33].
Large-scale mafic rocks as sills or dykes are widely distributed in northern Liaoning, generally trending east-west, and intruding into the Kangzhuangzi Formation, Guanmenshan Formation, Tongjiajie Formation, and Hutouling Formation with minor mafic intrusions into Neoarchean gneiss (Figure 1b).These mafic rocks can be subdivided into two belts: the northern belt extends from Guojia Gou to Xinlitun, and the southern belt extends from Chaihe Reservoir to Zengjiazhai.Additionally, sporadic outcrops of mafic rocks can be observed in the Xiongguantun area of Tieling City.They exhibit obvious contact metamorphic belt and chilled margins, with varying widths and abundant country rock xenoliths.The Precambrian basement in this area is covered by the Jurassic Qianwanling Formation, Nankangzhuang Formation, and Yingshugou Formation with unconformity (Figure 1b).Furthermore, through the 1:50,000 regional geological survey, a suite of twomica schist, marble, and basalt (partially pillow-shaped) rock assemblage was identified in Wangxiaopu Village, which is in fault contact with the surrounding Permian granite and is intruded by later quartz veins (Figure 1c).

Petrological Characteristics of Mafic Rocks
In this study, 4 geochronological samples and 28 geochemical samples were collected from the intrusions of diabase, gabbro sills, and basalt associated with marble in the Proterozoic strata for analysis, respectively.The sampling locations are shown in Figure 1 and Table 1.
Sample D1917, basalt, was collected from 500 m south of Yunpangou Village, Xiongguantun Town (Figure 1b).It crops out as a pillow structure, with a fine-grained contact margin between the pillows (Figure 2a), and intrudes into the Guanmenshan Formation.It is black to grey in color with a porphyritic texture, massive structure, and a matrix of microcrystalline texture (Figure 2b,c).The phenocrysts are composed of plagioclase (~5%) and pyroxene (~5%).Plagioclase, subhedral to euhedral columnar crystals ranging from 0.5-2 mm in diameter, has undergone chloritization, epidotization, and calcitization.Pyroxene, euhedral columnar crystals, with a particle size of 0.5-2 mm, has undergone alteration into amphibole and carbonation.The matrix consists of microcrystalline plagioclase (~55%) and pyroxene (~30%).Plagioclase is crystallized in subhedral columns with a particle size of 0.2-0.5 mm.Pyroxenes exhibit xenomorphic granular textures with a particle size of less than 0.2 mm.The accessory minerals are magnetite, apatite, etc., and the alteration minerals include chlorite, epidote, calcite, etc. (Figure 2c).Sample D2012, amphibolite, collected from the north of Wangxiaobao Village (Figure 1c), intruded by later felsic veins (Figure 2j), was wrapped in marble in a pillow shape in the field (Figure 2k), which displayed as the rock assemblage of oceanic islands.It is composed of primary minerals such as hornblende (~77%), plagioclase (~12%), and quartz (~6%) exhibiting a fine granular texture, columnar recrystallization texture, and gneissic structure (Figure 2l).Gray to green hornblende crystallizes as long columns with a particle size ranging from 0.05 to 85 mm.Plagioclase, subhedral-xenomorphic plate-columnarshaped, with a particle size of 0.12-0.50mm, shows obvious sericitization and clayification  Sample D1918, diabase, collected from northwest of Baiqizhai Town (Figure 1b), intrudes into the dolomite of the Guanmenshan Formation (Figure 2d) with a subhedral granular texture and massive structure (Figure 2e).The mineral components of the diabase are plagioclase (~55%), hornblende (~35%), biotite (~5%), and quartz (~3%) (Figure 2f).Plagioclase, with a subhedral columnar texture, and a polysynthetic twin, with a particle size of 0.5-2 mm and some particles of 2-3 mm, exhibits varying degrees of epidotization and zoidization.Hornblende occurs as brown-green, subhedral columnar crystals ranging from 0.2 to 2 mm in diameter, fading to light green hornblende in varying degrees.Biotite is brown, idiomorphic flaky, with a particle size of 0.2-2 mm, and exhibits varying degrees of chloritization showing a pseudomorphic or residual structure.Xenomorphic crystals of quartz fill interstices between plagioclase and hornblende grains with a particle size of <0.2 mm.Accessory minerals include magnetite, apatite, and sphene (Spn), with a content of about 2%.Alteration includes sericitization, chloritization, and epidotization (Figure 2f).Sample D1919, gabbro, collected from north of Huangqizhai Town (Figure 1b), intrudes into dolomite (Figure 2g,h).It shows a gabbroic texture, an embedded olivine texture, and a massive structure (Figure 2i), and consists of plagioclase (~55%), pyroxene (~30%), olivine (~8%), hornblende (~5%), and biotite (~2%).Plagioclase shows a subhedral columnar texture and a polysynthetic twin, with a main particle size of 0.2-2 mm and some particles of 2-5 mm and 5-8 mm, and exhibits zoisitization.Pyroxene, subhedral columnar crystals ranging from 0.2 to 2 mm in diameter, shows an embedded olivine texture containing granular olivine, and partially shows brown amphibole reaction edges.Olivine, with a xenomorphic granular texture and a particle size of 0.1-0.5 mm, is strongly altered to serpentine, microscale biotite, magnetite, etc., retaining its structure.Colorless to green hornblende, xenomorphic to granular in shape, shows amphibole cleavage with a particle size of 0.2-0.5 mm, and has undergone varying degrees of chloritization.Biotite is brown, idiomorphic flaky, with a particle size of 0.2-0.5 mm.The accessory minerals are magnetite, ilmenite, etc. (Figure 2i).Sample D2012, amphibolite, collected from the north of Wangxiaobao Village (Figure 1c), intruded by later felsic veins (Figure 2j), was wrapped in marble in a pillow shape in the field (Figure 2k), which displayed as the rock assemblage of oceanic islands.It is composed of primary minerals such as hornblende (~77%), plagioclase (~12%), and quartz (~6%) exhibiting a fine granular texture, columnar recrystallization texture, and gneissic structure (Figure 2l).Gray to green hornblende crystallizes as long columns with a particle size ranging from 0.05 to 85 mm.Plagioclase, subhedral-xenomorphic plate-columnar-shaped, with a particle size of 0.12-0.50mm, shows obvious sericitization and clayification with polysynthetic twin partially.Quartz, with a xenomorphic granular texture and a particle size of 0.04-0.30mm, shows a wavy extinction.A large amount of opaque dark minerals are distributed in ribbons and clumps, with a total content of about 5%.Alteration includes chloritization and epidotization (Figure 2l).

Zircons in These Mafic Rocks
Commonly, some researchers contend that zircons are exclusively observed in felsic rocks due to the gradual saturation of Zr (and Si) during magma evolution.Conversely, the origin of zircons from mafic rocks, particularly fine-grained basalt and diabase, is considered highly improbable [40,41].However, in recent research, some researchers believe that zircon crystallization in low-Zr mafic magmas is possible [42].They explored that possibility using 2D finite elements to model the crystallization of MORB melts confined in pores, and found that zircon-saturated volumes may form locally at the growing mineral-melt interfaces if the growth rate of a low Kd Zr mineral (<0.2) is much faster than the diffusion rate of the rejected Zr 4+ away into the melt, thus leading to the precipitation of zircon in low-Zr mafic magmas.Thus, zircon crystallization in low-Zr mafic magmas is perfectly possible under confined crystallization [42].Through microscopic identification, zircons were identified in thin sections of these four mafic rocks.The micrographs are as follows (Figure 3).
with polysynthetic twin partially.Quartz, with a xenomorphic granular texture and a particle size of 0.04-0.30mm, shows a wavy extinction.A large amount of opaque dark minerals are distributed in ribbons and clumps, with a total content of about 5%.Alteration includes chloritization and epidotization (Figure 2l).

Zircons in These Mafic Rocks
Commonly, some researchers contend that zircons are exclusively observed in felsic rocks due to the gradual saturation of Zr (and Si) during magma evolution.Conversely, the origin of zircons from mafic rocks, particularly fine-grained basalt and diabase, is considered highly improbable [40,41].However, in recent research, some researchers believe that zircon crystallization in low-Zr mafic magmas is possible [42].They explored that possibility using 2D finite elements to model the crystallization of MORB melts confined in pores, and found that zircon-saturated volumes may form locally at the growing mineral-melt interfaces if the growth rate of a low Kd Zr mineral (<0.2) is much faster than the diffusion rate of the rejected Zr 4+ away into the melt, thus leading to the precipitation of zircon in low-Zr mafic magmas.Thus, zircon crystallization in low-Zr mafic magmas is perfectly possible under confined crystallization [42].Through microscopic identification, zircons were identified in thin sections of these four mafic rocks.The micrographs are as follows (Figure 3).

Sample Preparation
Samples for geochronological analyses were first cleaned, crushed, and ground after being collected from the field.The zircon crystals were then separated from these samples using conventional heavy liquid and magnetic techniques at Langfang Yuneng Mineral Separation Co., Ltd. in Langfang, China.The separated zircons were carefully handpicked under a binocular microscope.To examine their internal structures, the selected zircons were embedded in epoxy resin, polished, and imaged using a scanning electron microscope with cathodoluminescence (CL) at the Beijing Gaonian Navigation Technology Limited Company in Beijing, China.CL images of four samples were obtained using a CAMECA SX51 microprobe (CAMRCA, Gennevilliers, France), operating at 50 kv and 15 nA.

Sample Preparation
Samples for geochronological analyses were first cleaned, crushed, and ground after being collected from the field.The zircon crystals were then separated from these samples using conventional heavy liquid and magnetic techniques at Langfang Yuneng Mineral Separation Co., Ltd. in Langfang, China.The separated zircons were carefully hand-picked under a binocular microscope.To examine their internal structures, the selected zircons were embedded in epoxy resin, polished, and imaged using a scanning electron microscope with cathodoluminescence (CL) at the Beijing Gaonian Navigation Technology Limited Company in Beijing, China.CL images of four samples were obtained using a CAMECA SX51 microprobe (CAMRCA, Gennevilliers, France), operating at 50 kv and 15 nA.

Zircon LA-ICP-MS U-Pb Isotope Dating
The LA-ICP-MS U-Pb isotope analyses were performed using an Agilent 7500a (Agilent Technologies Co., Ltd, Santa Clara, CA, USA) quadrupole ICP-MS with a UP-193 Solid-State Laser (193 nm, New Wave Research Inc., Shanghai, China).The laser spot size was set to 32 µm for most of the analyses, with a laser energy density of 10 J/cm 2 and a repetition rate of 8 Hz.The laser sampling procedure consisted of a 30 s blank, followed by a 30 s sampling ablation, and a 2 min sample-chamber flushing after the ablation.The ablated material was carried into the ICP-MS by a high-purity Helium gas stream with a flux of 1.15 L/min.The entire laser path was fluxed with Ar (600 mL/min) to ensure energy stability.The counting times were 20 ms for 204 Pb, 206 Pb, 207 Pb, and 208 Pb, 15 ms for 232 Th and 238 U, 20 ms for 49 Ti, and 6 ms for other elements.NIST 610 glass was used as an external standard and Si as an internal standard for calibrations in the zircon analyses.U-Pb isotope fractionation effects were corrected using zircon 91500 [43] as an external standard.Isotopic ratios and element concentrations of zircons were calculated using Glitter [44].Concordia ages and diagrams were obtained using Isoplot/Ex (3.0) [44].The common lead was corrected using LA-ICP-MS Common Lead Correction (ver.3.15), following the method of Andersen (2002) [45].The analytical data are presented on U-Pb Concordia diagrams with 1σ errors.The mean ages are weighted means at 95% confidence levels [44].The analyses of four samples were conducted at the Key laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Natural Resources.The data are listed in Table 2.

Major and Trace Element Analyses
After conducting major, trace, and rare earth element (REE) analyses for 28 samples, the results were obtained from the Northeast China Supervision and Inspection Center of Mineral Resources, Ministry of Natural Resources (Shenyang, China).The samples underwent petrographic examination and the altered rock surfaces were removed before being crushed and ground to 74 µm using an agate mill.The major element contents of the whole rock were determined using X-ray fluorescence spectrometry (XRF), with analytical precisions exceeding 2%.The trace element and REE contents were determined using X Series II ICP-MS from American Thermal Power Company in Salt Lake City Utah State.A total of 0.1 g of the sample was placed in a digestion tank, with 1 mL of concentrated nitric acid and 1 mL of hydrofluoric acid added.The digestion tank was placed in an oven and was heated up to 180 degrees for 10-12 h.After removal, it was heated at 120 degrees on an electric heating plate in an open environment.When about 2-3 mL of the digestion solution was left, it was heated up to 240 degrees.After redissolving, 0.5% dilute nitric acid was added to the mark for measurement.Twelve measurements were conducted on samples, with analytical precisions exceeding 5% for elements with contents of >10 µg/g, exceeding 8% for elements with contents of <10 µg/g, and 10% for the transition metals.The results are listed in Table 3.

Zircon U-Pb Geochronology
Sample D1917 is a basalt collected from Xiongguantun Town Yunpangou Village in Tieling City.Eighteen isotopic analyses were conducted on 18 zircons from this sample, and can be divided into three groups.Dark edges with different widths occurred around most zircons from the first group, which may be the proliferative edges formed by recrystallization during metamorphism.They are mostly self-shaped rhombohedral with diameters of 60-140 µm and length-width ratios of 1.5:1 to 3:1, generally black, clear oscillatory zoning (Figure 4), and the combination with their higher Th/U ratios (0.54-1.11), higher contents of total rare earth elements (REEs) (average contents of 585.65 µg/g), Nb, and Ta (average contents of 1.94 µg/g and 0.58 µg/g), and a significant positive Ce anomaly and negative Eu anomaly (Figure 5a), indicates a magmatic origin [46][47][48][49].Fifteen analyses yielded an upper intercept age of 2550 ± 16 Ma (n = 15, MSWD = 0.1), which is consistent with the 207 Pb/ 206 Pb weighted average age of 2544 ± 18 Ma (n = 9, MSWD = 3.5) (Figure 6a).The age of the first group is interpreted as the captured zircons from the surrounding Archean rocks.The second group is composed of two zircons (6,16) with diameters of 60-70 µm and length-width ratios around 2:1 (Figure 4), which combined with their evident internal structure and Th/U ratios (0.60, 0.67), higher REEs (average contents of 840.5 µg/g), Nb, and Ta (average contents of 1.65 µg/g and 0.53 µg/g), and a significant positive Ce anomaly and negative Eu anomaly (Figure 5a), indicate a magmatic origin [46][47][48][49].Two analyses yield a concordant weighted average 207 Pb/ 206 Pb age of 2154 ± 15 Ma (n = 2, MSWD = 0.93) (Figure 6a).Thus, this represents the magmatic crystallization age.Only one zircon (data 12) belongs to the third group.It exhibits rounded and embayed boundaries, largely homogeneous central regions with an unzoned internal structure (Figure 4) and low ratio of Th/U (0.08), lower contents of REEs (127.03 µg/g), Nb, and Ta (0.04 µg/g and 0.02 µg/g), and a flat Ce and Eu distribution (Figure 5a), which indicate a metamorphic zircon [46][47][48][49].This zircon has a concordant 207 Pb/ 206 Pb age of 1831 ± 13 Ma (Figure 6a) and is interpreted as the time of late metamorphism.
Minerals 2024, 14, x FOR PEER REVIEW 13 of 28 homogeneous central regions with an unzoned internal structure (Figure 4) and low ratio of Th/U (0.08), lower contents of REEs (127.03 µg/g), Nb, and Ta (0.04 µg/g and 0.02 µg/g), and a flat Ce and Eu distribution (Figure 5a), which indicate a metamorphic zircon [46][47][48][49].This zircon has a concordant 207 Pb/ 206 Pb age of 1831 ± 13 Ma (Figure 6a) and is interpreted as the time of late metamorphism.Only six zircons have been separated from the D1918 diabase sample, and they are euhedral stubby prisms (length/width ratios 1.5:1-3:1), clear and colorless to light-brown, ranging in size from 50 to 120 µm, with larger grains being fragments presumably broken during processing.In cathodoluminescence (CL) imaging, the internal structures vary from high-CL to low-CL contrast fine oscillatory zoning in both CL-bright and CL-dark grains (Figure 4).Combined with high Th/U ratios (0.60, 0.67), higher contents of REEs (average contents of 1271.34 µg/g), Nb, and Ta (average contents of 1.96 µg/g and 0.76 µg/g), and a significant positive Ce anomaly and negative Eu anomaly (Figure 5b), this indicates that the zircons in this sample are magmatic-derived zircon [46][47][48][49].Six analyses yielded an upper intercept age of 2203 ± 50 Ma (n = 6, MSWD = 5.7), which is consistent with the 207 Pb/ 206 Pb age (2209 ± 12 Ma) in the concordant line (Figure 6b).Thus, this age is interpreted as the time of magmatic emplacement.Zircons from the gabbro sample D1919 are euhedral prisms ranging in size from 50 to 130 µm with length/width ratios from 2:1 to 4:1 with some rounding of their apices.Although exhibiting a low contrast, CL imaging of the internal structure reveals dominantly fine oscillatory zoning (Figure 4).With the Th/U ratios ranging from 0.47 to 1.82, higher contents of REEs (average contents of 933.35 µg/g), Nb, and Ta (average contents of 2.97 µg/g and 1.19 µg/g), and a significant positive Ce anomaly and negative Eu anomaly (Figure 5c), these zircons were considered to be of magmatic origin [46][47][48][49].A total of 20 U-Pb isotopic analyses were conducted on 20 zircons from this sample, yielding an upper intercept 207 Pb/ 206 Pb age of 2055 ± 23 Ma (n = 20, MSWD = 3.0), which is consistent with the weighted average 207 Pb/ 206 Pb age of 2063 ± 7 Ma (n = 8, MSWD = 0.18) from eight zircons on the concordant line (Figure 6c).This age is interpreted as the crystallization age of the gabbro.
aly (Figure 5c), these zircons were considered to be of magmatic origin [46][47][48][49].A total of 20 U-Pb isotopic analyses were conducted on 20 zircons from this sample, yielding an upper intercept 207 Pb/ 206 Pb age of 2055 ± 23 Ma (n = 20, MSWD = 3.0), which is consistent with the weighted average 207 Pb/ 206 Pb age of 2063 ± 7 Ma (n = 8, MSWD = 0.18) from eight zircons on the concordant line (Figure 6c).This age is interpreted as the crystallization age of the gabbro.Sample D2012, which was collected from Wangxiaopu Village, yielded prismatic and sub-angular zircon ranging in size from 35 µm to 120 µm with length/width ratios from 1:1 to 4:1.The zircon grains contain numerous inclusions and are divided into two groups according to the CL images (Figure 4).The rounded or irregular cores and white rims are present in these grains from the first group and are CL-dark with no obvious zoning.The ratios of Th/U range from 0.48 to 0.98, with high contents of REEs (average contents of 232.59 µg/g), Nb, and Ta (average contents of 0.95 µg/g and 0.48 µg/g), and a significant positive Ce anomaly and negative Eu anomaly (Figure 5d) indicate a magmatic origin [46][47][48][49].Seven analyses were obtained from seven zircons, yielding a concordant 207 Pb/ 206 Pb age of 2428 ± 19 Ma (n = 7, MSWD = 0.05) (Figure 6d).Due to the small size of zircon, both the core and rim were collected during sampling, so this age represents a mixed age, and these zircons are interpreted as the captured zircons from the surrounding Archean rocks.The other groups of zircons generally have an evident internal structure with fine, oscillatory zoning and Th/U ratios (0.59-1.72), higher contents of REEs (average contents of 2736.25 µg/g), Nb and Ta (average contents of 13.40 µg/g and 1.81 µg/g), and a significant positive Ce anomaly and negative Eu anomaly (Figure 5d), which show the characteristics of magmatic origin zircon [46][47][48][49].Ten analyses were concordant to near-concordant and yielded an upper intercept 207 Pb/ 206 Pb age of 2017 ± 39 Ma (n = 10, MSWD = 0.01), and a weighted average 207 Pb/ 206 Pb age of 2018 ± 13 Ma (n = 10, MSWD = 0.02) was obtained from these same ten grains (Figure 6d).This age is interpreted as the magmatic protolith age.
In summary, all the zircons separated from mafic rock samples in northern Liaoning of the northern margin of the NCC record four distinct age groups including the first basalt (~2209 Ma (D1918)), the second diabase (~2154 Ma (D1917)), the third gabbro (~2063 Ma (D1919), and the forth amphibolite (~2018 Ma (D2012)), which represent the emplacement ages for the mafic rocks, respectively.

Major and Trace Element Geochemistry
The average SiO 2 content of the first three periods of mafic rocks is 49.27-50.33wt%, while the fourth amphibolite has a lower average SiO 2 content (46.39 wt%).The Al 2 O 3 content is moderate (13.22-14.86wt%).The total alkali content (K 2 O + Na 2 O) is moderate, with the lowest being 2.95 wt% in the first basalt and ranging from 3.93 to 5.08 wt% in the other three stages of mafic rocks.On the TAS diagram (Figure 7a), most of the samples belong to the gabbro, and on the R1-R2 diagram (Figure 7b), they fall within the gabbro and olivine-gabbro area.The K 2 O content of samples is very variable, with average values of 0.94 wt%, 0.42 wt%, 1.73 wt%, and 1.32 wt%, respectively.On the SiO 2 -K 2 O diagram, the samples belong to the calc-alkaline to high-K calc-alkaline series (Figure 7c), and Ta/Yb vs. Ce/Yb ratios allow an allocation to the calc-alkaline series (Figure 7d).The average Fe 2 O 3 content of the samples is 2.13-3.50wt%, with a high average FeO content of 9.41-11.90wt%, an average MgO content of 6.17-7.50wt%, and a low average content of TiO 2 (1.61-2.39wt%) and MnO (0.17-0.21 wt%).Mg # is low, ranging from 43.32 to 52.02, which is much lower than that of primary basaltic rocks (Mg # = 70 [50]).The first basalt and the fourth amphibolite have a relatively high CaO content (9.52-10.00wt%) and low P 2 O 5 content (0.18-0.19 wt%), while the other two periods of mafic rocks have a relatively low CaO content (6.90-7.17wt%) and high P 2 O 5 content (0.25-0.39 wt%).
There are differences in the characteristics of rare earth elements and trace elements in these mafic rocks.The total contents of rare earth elements (REEs) in the first basalt and the fourth amphibolite are low, with average contents of 83 µg/g and 86 µg/g, respectively.On the chondrite-normalized rare earth element (REE) diagram, they exhibit similar characteristics to ocean island basalt (OIB), with a right-skewed smooth curve (Figure 8a), indicating an enrichment of light rare earth elements (LREEs) and a flat distribution pattern of heavy rare earth elements (HREEs).The differentiation between LREEs and HREEs is not significant, with an average (La/Yb) N value of 2.55-4.78and a slight positive europium anomaly (Eu/Eu* = 0.98-1.06)(Figure 8a).In the primitive mantle-normalized trace element spider diagram, these two mafic rocks demonstrate similar characteristics, with an enrichment of large ion lithophile elements (LILEs) (Rb, Ba, K, etc.), depletion of high field strength elements (HFSEs) (Th, U, Nb, Ta, Zr, Ti, etc.), and negative anomalies of Sr and P, as well as slight positive anomalies of Zr and Hf (Figure 8b).The contents of Cr (with average contents of 106 µg/g and 129 µg/g), Co (with average contents of 51.2 µg/g and 48.4 µg/g), and Ni (with average contents of 73.6 µg/g and 98 µg/g) in these two mafic rocks are higher than normal mid-ocean ridge basalt (N-MORB).There are differences in the characteristics of rare earth elements and trace elements in these mafic rocks.The total contents of rare earth elements (REEs) in the first basalt and the fourth amphibolite are low, with average contents of 83 µg/g and 86 µg/g, respectively.On the chondrite-normalized rare earth element (REE) diagram, they exhibit similar characteristics to ocean island basalt (OIB), with a right-skewed smooth curve (Figure 8a), indicating an enrichment of light rare earth elements (LREEs) and a flat distribution pattern of heavy rare earth elements (HREEs).The differentiation between LREEs and HREEs is not significant, with an average (La /Yb)N value of 2.55-4.78and a slight positive europium anomaly (Eu/Eu* = 0.98-1.06)(Figure 8a).In the primitive mantle-normalized trace element spider diagram, these two mafic rocks demonstrate similar characteristics, with an enrichment of large ion lithophile elements (LILEs) (Rb, Ba, K, etc.), depletion of high field strength elements (HFSEs) (Th, U, Nb, Ta, Zr, Ti, etc.), and negative anomalies of Sr and The second diabase and the third gabbro show similar average REE contents, which are 119.28µg/g and 121.41 µg/g, respectively, and are between OIB and N-MORB [55].On the chondrite-normalized REE diagram, the samples show a right-skewed curve (Figure 8a), indicating an enrichment of LREEs and a flat pattern of HREEs.There is a slight differentiation between them, with average (La/Yb) N values of 5.44 and 7.32, respectively.The diabase shows a slight negative europium anomaly (Eu/Eu* = 0.72-0.88),while the gabbro exhibits a positive europium anomaly (Eu/Eu* = 1.31-1.37)(Figure 8a).In the primitive Minerals 2024, 14, 717 19 of 30 mantle-normalized trace element spider diagram, the diabase is enriched in LILEs (Ba, etc.), relatively depleted in Rb and HFSEs (Th, U, Nb, Ta, Zr, Hf, Ti, etc.), and shows slight positive anomalies of K, Sr, and P, and Eu shows negative anomalies (Figure 8b); the gabbro shows the enrichment of LILEs (Rb, Ba, K), depletion of HFSEs (Th, U, Nb, Ta, Zr, Hf, etc.), and a positive anomaly of Eu (Figure 8b).The contents of Cr (with average contents of 123 µg/g and 113 µg/g), Co (with average contents of 46.8 µg/g and 62.7 µg/g), and Ni (with average contents of 71.4 µg/g and 74.0 µg/g) in these two mafic rocks are slightly higher than N-MORB.
Minerals 2024, 14, x FOR PEER REVIEW 17 of 28 P, as well as slight positive anomalies of Zr and Hf (Figure 8b).The contents of Cr (with average contents of 106 µg/g and 129 µg/g), Co (with average contents of 51.2 µg/g and 48.4 µg/g), and Ni (with average contents of 73.6 µg/g and 98 µg/g) in these two mafic rocks are higher than normal mid-ocean ridge basalt (N-MORB).The second diabase and the third gabbro show similar average REE contents, which are 119.28µg/g and 121.41 µg/g, respectively, and are between OIB and N-MORB [55].On the chondrite-normalized REE diagram, the samples show a right-skewed curve (Figure 8a), indicating an enrichment of LREEs and a flat pattern of HREEs.There is a slight differentiation between them, with average (La /Yb)N values of 5.44 and 7.32, respectively.The diabase shows a slight negative europium anomaly (Eu/Eu* = 0.72-0.88),while the gabbro exhibits a positive europium anomaly (Eu/Eu* = 1.31-1.37)(Figure 8a).In the primitive mantle-normalized trace element spider diagram, the diabase is enriched in LILEs (Ba, etc.), relatively depleted in Rb and HFSEs (Th, U, Nb, Ta, Zr, Hf, Ti, etc.), and shows slight positive anomalies of K, Sr, and P, and Eu shows negative anomalies (Figure 8b); the gabbro shows the enrichment of LILEs (Rb, Ba, K), depletion of HFSEs (Th, U, Nb, Ta, Zr, Hf, etc.), and a positive anomaly of Eu (Figure 8b).The contents of Cr (with average contents of 123 µg/g and 113 µg/g), Co (with average contents of 46.8 µg/g and 62.7 µg/g), and Ni (with average contents of 71.4 µg/g and 74.0 µg/g) in these two mafic rocks are slightly higher than N-MORB.

Constraints on the Age of Proterozoic Strata in Northern Liaoning
The Proterozoic strata, exposed in northern Liaoning, are placed in the Changcheng, Jixian, and Nanhua System of the Meso-Neoproterozoic, according to the comparison of rock assemblage with the Proterozoic strata in western Liaoning [33].However, after field investigation, this suite of strata has certain differences from the strata of Mesoproterozoic in western Liaoning.The Proterozoic strata in northern Liaoning has generally undergone metamorphism, with the lithology consisting of dolomite, meta-sandstone, meta-siltstone, and slate.These characteristics of rock composition and low-grade metamorphism are similar to those of the North Liaohe Groups and Laoling Groups in JLJB [56].The mafic rocks studied in this paper intrude into the Kangzhuangzi Formation, Guanmenshan For-

Constraints on the Age of Proterozoic Strata in Northern Liaoning
The Proterozoic strata, exposed in northern Liaoning, are placed in the Changcheng, Jixian, and Nanhua System of the Meso-Neoproterozoic, according to the comparison of rock assemblage with the Proterozoic strata in western Liaoning [33].However, after field investigation, this suite of strata has certain differences from the strata of Mesoproterozoic in western Liaoning.The Proterozoic strata in northern Liaoning has generally undergone metamorphism, with the lithology consisting of dolomite, meta-sandstone, meta-siltstone, and slate.These characteristics of rock composition and low-grade metamorphism are similar to those of the North Liaohe Groups and Laoling Groups in JLJB [56].The mafic rocks studied in this paper intrude into the Kangzhuangzi Formation, Guanmenshan Formation, Tongjiajie Formation, and Hutouling Formation in northern Liaoning.They are considered to have formed in the Early Triassic according to whole-rock K-Ar dating results of 238.2 Ma [33].
However, the credibility of this chronology result is doubted: (1) In northern Liaoning, these mafic rocks have intruded into the earlier strata with objective contact metamorphic belts, rather than the Neoproterozoic strata (Figure 1b).( 2) Partial pillow-shaped mafic rocks, associated with marble and dolomite, are in tectonic contact with the surrounding Permian granite (Figure 1c).( 3) Metamorphism is widely developed in these mafic rocks.Pyroxene has undergone metamorphism into amphibole and carbonation or with brown amphibole reaction edges partially.Plagioclase has undergone epidotization, zoisitization, etc. Olivine has intensely metamorphosed into microscale biotite, magnetite, etc. Hornblende has undergone chloritization.Some mafic rocks have metamorphosed into amphibolite (Figure 2k,l).
Four zircon U-Pb ages have been obtained from the mafic rocks in northern Liaoning, ~2209 Ma, ~2154 Ma, ~2055 Ma, and ~2018 Ma, recording four periods of mafic magmatic activity.These results indicate that the emplacement age of the mafic rocks is the Paleoproterozoic rather than the Early Triassic, which means the intruded strata by, or associated with, the mafic rocks were formed in the Paleoproterozoic or earlier.Combined with the unconformity of overlying the Neoproterozoic granite, the formation age of these strata should be corrected to Paleoproterozoic.5.1.2.Paleoproterozoic Mafic Magmatic Events in the Eastern Segment of the NCC In this study, four episodes of mafic magmatic activities have been identified in northern Liaoning.Meanwhile, a 2118 Ma metamorphic basic dyke has recently been reported in the Qingyuan area [30].The magmatic activities of mafic rocks in the eastern segment of the NCC are primarily concentrated in the JLBT and persist from approximately 2209 Ma to around 1820 Ma.The rock assemblage with this activity includes basalt, diabase, gabbro, and amphibolite, which are found as intrusions, sills, dykes, or strata interlayers coexisting with Paleoproterozoic meta-sedimentary and metamorphic rocks.Based on previous studies and new data obtained in this study, a chronological framework for these Paleoproterozoic mafic magmatic activities in the eastern segment of the NCC has been established.This framework provides a timeline of when these events occurred, helping scientists understand the sequence of geological processes during this time (Figure 8, Table 4).In the eastern segment of the NCC, the Paleoproterozoic mafic rocks underwent four events of magmatism: The period from 2210 to 2100 Ma was marked by an especially intense activity of mafic magmatism.This mafic magmatic event started around 2209 Ma and persisted until 2100 Ma, with two significant peaks occurring at approximately 2162 Ma and 2108 Ma, respectively.The geological implications of this phenomenon suggest the potential onset and continuous development of ocean opening processes, commencing around 2209 Ma and extending until 2100 Ma when the ocean basin reached its maximum extent.From 2100 to 2000 Ma, the mafic magmatic activity gradually decreased, possibly indicating the onset of subduction and reduced basaltic magma activity in a compressional tectonic setting.Occasional mafic magmatic activity was observed from 2000 to 1900 Ma, which may be associated with a continent-continent collision and crustal thickening after 2000 Ma, resulting in reduced magmatic activity.After 1900 Ma, there was an increase in mafic magmatic activities.This could be attributed to the post-orogenic extensional phase.In this tensional tectonic setting, a certain scale of mafic magmatism occurred.However, due to the thicker crust at this time compared to earlier periods, the intensity of mafic magmatism during this stage was not as strong as that observed in the initial phase (Figure 9, Table 4).

Source Characteristics and Genesis of the Paleoproterozoic Mafic Rocks
The ratio of some incompatible elements is stable during partial melting and fractional crystallization processes, which can largely reflect the characteristics of the source.For example, since Nb and Ta have similar valence states and ionic radii, their ratios are also comparable in igneous rocks from the same source.Similarly, Zr and Hf have similar valence states, ionic radii, and distribution coefficients in various minerals, so similar ratios and trends can be observed in rocks derived from the same origin.The ratios of Nb/Ta, Nb/La, Ta/Th, Ce/Pb, La/Pb, Nb/U, Zr/Nb, and Zr/Hf in the four periods of mafic rocks all show differences (Table 3), indicating that they come from different sources.These mafic rocks have undergone medium-to high-grade metamorphism and later experienced alteration processes such as sericitization, chloritization, and epidotization.During the process, highly active elements may migrate due to changing conditions, while REEs and HFSEs remain stable [67].These elements are used to analyze and discuss the magma series, genesis, and source characteristics of metamorphic rocks [67].
Generally, mafic rocks originate from the mantle source [68].The Paleoproterozoic mafic rocks are characterized by the enrichment of LREEs and LILEs, depletion of HREEs and HFSEs such as Nb, Ta, Ti, and Zr, and a slight negative or positive Eu anomaly, indicating characteristics of a mantle source [68].The REE and trace element patterns of the Paleoproterozoic mafic rocks are different from N-MORB and E-MORB, but similar to OIB (Figure 8a,b), suggesting their origin from the lithospheric mantle rather than the asthenospheric mantle [68].On the Nb/Th-Zr/Nb diagram, the first basalt is located between the primitive mantle, enriched mantle, and recycled slab, while the second diabase and fourth amphibolite are located at the transitional zone between the primitive mantle and recycled subduction slab, and the third gabbro is relatively close to the enriched mantle (Figure 10a).This indicates that the sources of these mafic rocks are transitional mantle.[71].(e,f) After [72].
Due to the similar distribution coefficients, trace elements are difficult to fractionate during partial melting or fractional crystallization; they are usually used to reflect the nature of the parent magma in the magma source [70].The first basalt shows flat patterns for REEs, with a low LREE/HREE ratio, indicating that there may be no residual garnet in the source area (Figure 10b,c).On the La/Yb-Yb diagram, it is located at the 3-4% melting position of the Amphibole-bearing spinel lherzolite melting curve (Figure 10b), indicating the presence of residual amphibole and spinel in the magma source.The other stages of mafic rocks show similar patterns of enrichment in LREEs, relatively flat patterns of HREEs, high LREE/HREE ratios, and low Yb contents, indicating the presence of residual garnet in the source (Figure 10b,c).On the La/Yb-Yb diagram, they are located near the Garnet spinel lherzolite melting curve (Figure 10b), indicating the residual garnet and spinel in the magma source.The garnet to spinel ratio of the diabase is between 50:50 and 25:75, and the ratio of the gabbro and amphibolite is between 50:50 and 70:30 (Figure 10b).Differing from the partial melting of spinel lherzolite, during the partial melting process of different proportions of clinopyroxene and garnet, the properties of La/Sm and Sm/Yb show different characteristics, with Sm/Yb not changing with the decrease in La/Sm [73].On the (La/Sm)N-( Sm/Yb)N diagram (Figure 10c), consistent with the above conclusion, there is no garnet in the source, and the first basalt is the product of 3-4% melting of amphibole-bearing spinel lherzolite.The other three stages of mafic rocks are products of partial melting of garnet-spinel lherzolite with a ratio of clinopyroxene to garnet of 6:1 to 5:2, only with different degrees of partial melting (Figure 10c, [74]).The diabase, gabbro, and amphibolite show partial melting degrees of about 3%, 1%-2%, and 3%-5%, respectively (Figure 10c).All mafic rocks show negative Sr anomalies (Figure 10b), indicating the possible residual plagioclase in the source.
The mafic rocks exhibit similar enrichments in LREEs and LILEs such as Rb, Ba, and K, as well as depletions in HFSEs such as Nb, Ta, Zr, Hf, Ti, and P. In the La/Nb-Ba/Nb diagram (Figure 10d), the third gabbro falls within the area of island arc volcanic rocks, while the other mafic rocks fall within the transitional zone between Dupal OIB and arc volcanic rocks.This could be related to the introduction of fluids or involvement of mantle  [71].(e,f) After [72].
Due to the similar distribution coefficients, trace elements are difficult to fractionate during partial melting or fractional crystallization; they are usually used to reflect the nature of the parent magma in the magma source [70].The first basalt shows flat patterns for REEs, with a low LREE/HREE ratio, indicating that there may be no residual garnet in the source area (Figure 10b,c).On the La/Yb-Yb diagram, it is located at the 3-4% melting position of the Amphibole-bearing spinel lherzolite melting curve (Figure 10b), indicating the presence of residual amphibole and spinel in the magma source.The other stages of mafic rocks show similar patterns of enrichment in LREEs, relatively flat patterns of HREEs, high LREE/HREE ratios, and low Yb contents, indicating the presence of residual garnet in the source (Figure 10b,c).On the La/Yb-Yb diagram, they are located near the Garnet spinel lherzolite melting curve (Figure 10b), indicating the residual garnet and spinel in the magma source.The garnet to spinel ratio of the diabase is between 50:50 and 25:75, and the ratio of the gabbro and amphibolite is between 50:50 and 70:30 (Figure 10b).Differing from the partial melting of spinel lherzolite, during the partial melting process of different proportions of clinopyroxene and garnet, the properties of La/Sm and Sm/Yb show different characteristics, with Sm/Yb not changing with the decrease in La/Sm [73].On the (La/Sm) N -(Sm/Yb) N diagram (Figure 10c), consistent with the above conclusion, there is no garnet in the source, and the first basalt is the product of 3%-4% melting of amphibole-bearing spinel lherzolite.The other three stages of mafic rocks are products of partial melting of garnet-spinel lherzolite with a ratio of clinopyroxene to garnet of 6:1 to 5:2, only with different degrees of partial melting (Figure 10c [74]).The diabase, gabbro, and amphibolite show partial melting degrees of about 3%, 1%-2%, and 3%-5%, respectively (Figure 10c).All mafic rocks show negative Sr anomalies (Figure 10b), indicating the possible residual plagioclase in the source.
The mafic rocks exhibit similar enrichments in LREEs and LILEs such as Rb, Ba, and K, as well as depletions in HFSEs such as Nb, Ta, Zr, Hf, Ti, and P. In the La/Nb-Ba/Nb diagram (Figure 10d), the third gabbro falls within the area of island arc volcanic rocks, while the other mafic rocks fall within the transitional zone between Dupal OIB and arc volcanic rocks.This could be related to the introduction of fluids or involvement of mantle components, indicating that metasomatic processes occurred in the source [75].Thus, is the metasomatism caused by melts or fluids?The mafic rocks show high contents of Cr (average content of 106 µg/g, 123 µg/g, 113 µg/g, and 129 µg/g), Co (average content of 51.2 µg/g, 46.8 µg/g, 62.7 µg/g, and 48.4 µg/g), and Ni (average content of 73.6 µg/g, 71.4 µg/g, 74.0 µg/g, and 98 µg/g) compared to N-MORB, indicating that their source has undergone melt metasomatism, which is consistent with the trends shown in the Th/Zr-Nb/Zr diagram (Figure 10e).When the mantle-derived magma interacts with crustal material on the subducting plate and subducted oceanic crust, the magma generally exhibits low Na 2 O, P 2 O 5 , and TiO 2 contents, positive anomalies of Nb, Ta, and Ti [76], or low Ce/Th (≈8), low Ba/Th (≈111), and obvious negative Ce anomaly [77].The geochemical characteristics of these four episodes of mafic rocks are as follows: The first basalt exhibits a low Ce/Th (14.32), low Ba/Th (114.74),low Na 2 O, P 2 O 5 , and TiO 2 contents, negative anomalies of Nb, Ta, and Ti (Figure 8b), and a slight negative Ce anomaly (average Ce/Ce* = 0.94).The second diabase exhibits high Na 2 O, P 2 O 5 , and TiO 2 contents and positive anomalies of Nb, Ta, and Ti (Figure 8b), with low Ba/Th (114.74) but high Ce/Th (24.47), and a slightly negative Ce anomaly (average Ce/Ce* = 0.93).The third gabbro exhibits high Na 2 O, P 2 O 5 , and TiO 2 contents, low Ce/Th (14.32), negative anomalies of Nb and Ta, no anomaly in Ti (Figure 8b), high Ba/Th (371.73),high Ce/Th (21.33), and a slightly negative Ce anomaly (average Ce/Ce* = 0.91).The fourth amphibolite exhibits a low Ba/Th (110.08) and a positive anomaly in Ti, but low Na 2 O, P 2 O 5 , and TiO 2 contents, negative anomalies of Nb and Ta (Figure 8b), high Ce/Th (29.47), and a slightly negative Ce anomaly (average Ce/Ce* = 0.97).These characteristics indicate that all four stages of mafic rocks have undergone dual metasomatism by both fluid and melt, which is consistent with the trends shown in the Th/Zr-Nb/Zr and Th/Yb-Ba/La diagrams (Figure 10e,f).The third gabbro has undergone greater fluid metasomatism, while the others are mainly influenced by melt metasomatism.
Generally, mantle-derived magma may undergo assimilation and contamination with crustal materials during its ascent and emplacement.The geochemical data of the mafic rocks show moderate potassium and alkali contents (Figure 7c), low Mg # (Mg # = 43.32-52.02),enrichment in LILEs (Rb, Ba, K, etc.), and depletion in HFSEs (Nb, Ta, Zr, etc.).These characteristics suggest that the magma may have undergone contamination by crustal materials [78] [79]), and lower than those of mid-ocean ridge basalts (MORB) and primitive mantle (Nb/Ta = 17.7,Zr/Hf = 36.1 [55]), indicating the influence of crustal contamination on the mafic magmas.The La/Sm is often used to reflect the degree of crustal contamination [80].A higher La/Sm ratio often suggests a greater influence from crustal components, while a lower ratio may indicate a more pristine mantle signature [80].The average La/Sm ratios of the four episodes of mafic rocks are 2.95, 3.11, 4.03, and 2.75, respectively, indicating the incorporation of continental crustal material during magma ascent.The intensity of crustal contamination is highest in the third gabbro, followed by the first basalt and second diabase, and weakest in the fourth amphibolite.
In conclusion, the Paleoproterozoic mafic rocks originated from a transitional mantle source.The first basalt, containing residual hornblende and spinel in the source, is the product of 3%-4% partial melting of amphibole-bearing spinel lherzolite.The source of the other three stages of mafic rocks contains residual garnet and spinel, and experienced partial melting of garnet-spinel lherzolite with a clinopyroxene to garnet ratio ranging from 6:1 to 5:2.The garnet to spinel ratios in these three mafic rocks range from 50:50 to 25:75 and 50:50 to 70:30, with partial melting degrees of 3%, 1%-2%, and 3%-5%, respectively.During the ascent and emplacement of all mafic magmas, they were contaminated by crustal material and incorporated continental crustal material.
In this study, the third gabbro exhibits high TiO 2 , Nb, and Ta contents (averages of 2.17 wt%, 9.42 µg/g, and 0.63 µg/g, respectively), as well as high Ti/Y and Ti/V ratios (averages of 587 and 42.5, respectively).This may be attributed to the influence of melt contamination, resulting in excessively high TiO 2 contents.The average trace element ratios of Nb/La = 0.43, Hf/Ta = 5.57, La/Ta = 34.87,and Th/Ta = 3.19 indicate that it differs from WPB and MORB but shares similarities with continental arc basalts (CAB) [53,82].The gabbro also exhibits characteristics of island arc volcanic rocks in the La/Nb-Ba/Nb diagram (Figure 10d).Furthermore, in the Hf/3-Th-Nb/16 triangular diagram and Nb/Yb-Th/Yb diagram, the gabbro falls within the continental arc region (Figure 11a,b).These characteristics suggest that the gabbro was formed in an island arc environment.
In this study, the third gabbro exhibits high TiO2, Nb, and Ta contents (averages of 2.17 wt%, 9.42 µg/g, and 0.63 µg/g, respectively), as well as high Ti/Y and Ti/V ratios (averages of 587 and 42.5, respectively).This may be attributed to the influence of melt contamination, resulting in excessively high TiO2 contents.The average trace element ratios of Nb/La = 0.43, Hf/Ta = 5.57, La/Ta = 34.87,and Th/Ta = 3.19 indicate that it differs from WPB and MORB but shares similarities with continental arc basalts (CAB) [53,82].The gabbro also exhibits characteristics of island arc volcanic rocks in the La/Nb-Ba/Nb diagram (Figure 10d).Furthermore, in the Hf/3-Th-Nb/16 triangular diagram and Nb/Yb-Th/Yb diagram, the gabbro falls within the continental arc region (Figure 11a,b).These characteristics suggest that the gabbro was formed in an island arc environment.
The other three phases of mafic rocks show high TiO2 (averages of 1.61 wt%, 2.39 wt%, 1.94 wt%), Nb (averages of 13 µg/g, 21 µg/g, 11.2 µg/g), and Ta (averages of 0.83 µg/g, 1.28 µg/g, 0.76 µg/g) contents.The average trace element ratios of Nb/La (averages of 1.01, 1.08, 0.89), Ti/Y (averages of 351, 558, 587), Ti/V (averages of 30.01, 39.5, 43.4), Hf/Ta (averages of 4.23, 3.78, 4.82), La/Ta (averages of 15.4, 15.3, 16.6), and Th/Ta (averages of 2.42, 1.42, 1.33) indicate that they are similar with WPB and MORB.They also share similar characteristics in the Hf/3-Th-Nb/16 triangular diagram (Figure 11a).The first basalt and the second diabase intrude into the limestone as sills, while the fourth amphibolite is surrounded by marble.These field features are similar to the rock assemblage of oceanic islands or seamounts.The geochemical characteristics of these three phases of mafic rocks are similar to OIB.In the La/Nb-Ba/Nb diagram, they mainly originate from the Dupal OIB and transition to arc volcanic rocks (Figure 10d), and in the Nb/Yb-Th/Yb diagram, they fall on the MORB-OIB trend line, indicating that these three phases of mafic rocks were formed in an oceanic island environment (Figure 11b).The other three phases of mafic rocks show high TiO 2 (averages of 1.61 wt%, 2.39 wt%, 1.94 wt%), Nb (averages of 13 µg/g, 21 µg/g, 11.2 µg/g), and Ta (averages of 0.83 µg/g, 1.28 µg/g, 0.76 µg/g) contents.The average trace element ratios of Nb/La (averages of 1.01, 1.08, 0.89), Ti/Y (averages of 351, 558, 587), Ti/V (averages of 30.01, 39.5, 43.4), Hf/Ta (averages of 4.23, 3.78, 4.82), La/Ta (averages of 15.4, 15.3, 16.6), and Th/Ta (averages of 2.42, 1.42, 1.33) indicate that they are similar with WPB and MORB.They also share similar characteristics in the Hf/3-Th-Nb/16 triangular diagram (Figure 11a).The first basalt and the second diabase intrude into the limestone as sills, while the fourth amphibolite is surrounded by marble.These field features are similar to the rock assemblage of oceanic islands or seamounts.The geochemical characteristics of these three phases of mafic rocks are similar to OIB.In the La/Nb-Ba/Nb diagram, they mainly originate from the Dupal OIB and transition to arc volcanic rocks (Figure 10d), and in the Nb/Yb-Th/Yb diagram, they fall on the MORB-OIB trend line, indicating that these three phases of mafic rocks were formed in an oceanic island environment (Figure 11b).
The formation mechanism of the Paleoproterozoic orogenic belt (JLJB) in the eastern segment of the NCC has been a subject of ongoing debate and differing interpretations among geologists [4,5,8,9,[13][14][15][16][17][18][19][20]30,84].One view suggests that it resulted from subduction and continental collision from the northern plate of the Longgang block, with the JLJB forming in an arc-back basin, and the northern margin of the NCC belonging to a continental margin sedimentary environment [4,5].Previous studies on high-pressure granulite metamorphic rocks in the northern margin of the NCC proposed the existence of an eastwest striking Paleoproterozoic orogenic belt (IMNHB), which may have formed during the assembly of the Columbia supercontinent, influenced by the closure, disappearance, and collision of the northern ocean of the NCC [4,5,8,9].It is believed that this orogenic belt extended from Hebei to northern Liaoning [9].The Paleoproterozoic mafic magmatic events and metamorphism in the Hebei-Northern Liaoning area also support the existence of this orogenic belt [30,84].The mafic rocks identified in this study were formed in an oceanic island and island arc tectonic setting, indicating the existence of subduction, and the extinction of ocean and continent-continent collision in the north of the Longgang Block during the Paleoproterozoic, which is consistent with Kusky's viewpoint.
The debate between these two views lies in whether the subduction occurred in the south or north of the Longgang Block.The geochronological framework of mafic magmatic activity in the eastern segment of the NCC shows that there were similar mafic magmatic activities in the northern and southern parts of the Longgang Block during the Paleoproterozoic.Therefore, the northern and southern parts of the Longgang Block may have been in two different tectonic domains in the Paleoproterozoic.This issue requires further research in the future.

Conclusions
(1) Zircon U-Pb ages (2209 ± 12 Ma, 2154 ± 15 Ma, 2063 ± 7 Ma, 2018 ± 13 Ma) and metamorphic ages (1835 ± 13 Ma) have been obtained from mafic rocks in northern Liaoning, which constrained the formation age of the Proterozoic strata to be Paleoproterozoic.Based on previous data, the Paleoproterozoic mafic magmatic activities in the eastern segment of the NCC can be classified into four stages: the most frequent activity occurred between 2210 and 2100 Ma, gradually decreased between 2100 and 2000 Ma, with occasional activities between 2000 and 1900 Ma, and increased activities after 1900 Ma.
(2) Geochemical characteristics reveal that the four stages of mafic rocks belong to the calc-alkaline series and originated from transitional mantle.During the process of magma ascent and emplacement, the magma underwent contamination by crustal materials.The first basalt contains residual hornblende and spinel in the source, and is a product of 3-4% partial melting of amphibole-bearing spinel lherzolite.The other mafic rocks contain residual garnet and spinel in the source, and experienced partial melting of garnet-spinel lherzolite with a clinopyroxene to garnet ratio ranging from 6:1 to 5:2.The proportions of garnet to spinel range from 25:75 to 70:30, and the degrees of partial melting are 3%, 1%-2%, and 3%-5%, respectively.
(3) Trace element data indicate that the third gabbro exhibits characteristics of continental island arc basalts, suggesting it was formed in an island arc environment.The other three stages of mafic rocks originated from the Dupal OIB and formed in an oceanic island environment.The identification of Paleoproterozoic magmatic and subsequent

Figure 4 .
Figure 4. Cathodoluminescence (CL) images of the selected zircons from the mafic rocks in northern Liaoning.The circles on zircons represent analyzed spots.Figure 4. Cathodoluminescence (CL) images of the selected zircons from the mafic rocks in northern Liaoning.The circles on zircons represent analyzed spots.

Figure 4 .
Figure 4. Cathodoluminescence (CL) images of the selected zircons from the mafic rocks in northern Liaoning.The circles on zircons represent analyzed spots.Figure 4. Cathodoluminescence (CL) images of the selected zircons from the mafic rocks in northern Liaoning.The circles on zircons represent analyzed spots.

Figure 4 .
Figure 4. Cathodoluminescence (CL) images of the selected zircons from the mafic rocks in northern Liaoning.The circles on zircons represent analyzed spots.

Figure 6 .
Figure 6.Concordia diagrams for zircons analysed from mafic rocks in the northern Liaoning.

Figure 6 .
Figure 6.Concordia diagrams for zircons analysed from mafic rocks in the northern Liaoning.

Figure 8 .
Figure 8. Chondrite-normalized rare earth element patterns (a) and primitive mantle-normalized trace element spider diagram (b) for the mafic rocks from northern Liaoning.(The values of chondrite and primitive mantle are from [55].)

Figure 8 .
Figure 8. Chondrite-normalized rare earth element patterns (a) and primitive mantle-normalized trace element spider diagram (b) for the mafic rocks from northern Liaoning.(The values of chondrite and primitive mantle are from [55]).

Figure 9 .
Figure 9. Age histogram of mafic rocks from the eastern segment of the NCC.

Figure 9 .
Figure 9. Age histogram of mafic rocks from the eastern segment of the NCC.

Table 1 .
Location and lithology for the magmatic rocks from the Kaiyuan Area, North Liaoning.

Table 2 .
LA-ICP-MS zircon U-Pb dating data of mafic rocks from northern Liaoning.

Table 4 .
Summary of geochronological data of Paleoproterozoic mafic rocks in the eastern segment of the NCC.

Table 4 .
Summary of geochronological data of Paleoproterozoic mafic rocks in t of the NCC.
. The Nb/Ta (average values of 15.63, 16.53, 15.01, and 15.74) and Zr/Hf (average values of 30.07, 31.88,31.04, and 33.53) of the mafic rocks are similar to the values of the continental crust in eastern China (Nb/Ta = 15.38,Zr/Hf = 35.56,according to